title: Helical-core fiber analog of a quarter-wave plate for orbital angular momentum.
abstract: We have studied the effect of a twist defect on the conversion of the fundamental mode (FM) into an optical vortex (OV) in a helical-core fiber (HCF). We have shown that if such a twist defect is situated in the middle of the HCF, which converts the FM into an OV, such a fiber system can continuously change the orbital angular momentum (OAM) of the output field from 0 to 1 (in a.u.). This control of the OAM is achieved by variation of the twist angle. In this action upon the OAM, this system has analogy with the quarter-wave plate, which is able to change the spin angular momentum. We also introduced the generalized Stokes parameters (SPs) and Poincaré sphere to visualize evolution of the superposition of states with zero and nonzero OAM. Connection of SPs with geometric characteristics of the location of singularity is made.
authors.affiliation: Taurida National V.I. Vernadsky University, Vernadsky Prospekt, 4, Simferopol, Crimea 95007, Ukraine.
authors.forename: C N
authors.initials: CN
authors.lastname: Alexeyev
authors.forename: B P
authors.initials: BP
authors.lastname: Lapin
authors.forename: A V
authors.initials: AV
authors.lastname: Volyar
authors.forename: M A
authors.initials: MA
authors.lastname: Yavorsky